1. Field of the Invention
The present invention relates generally to inert gas welding. More particularly, the invention is directed to purge dam constructions for retaining purge gas around a weld zone.
2. Description of Prior Art
By way of background, inert gas welding is a species of arc welding in which the molten weld pool is shielded from atmospheric contamination and oxidation by bathing it with an inert gas, such as Argon, or a mixture of Helium and Argon. Popular examples of inert gas welding include TIG (Tungsten Inert Gas) welding and MIG (Metal Inert Gas) welding.
When welding together pipes and other enclosed structures using inert gas welding, it is important to purge the interior of the pipe or structure in the vicinity of the weld zone to prevent corrosion and the formation of oxides on the interior side of the weld pool. Purge dams are conventionally used for this purpose. For example, when butt-welding or socket-welding the ends of two pipe sections to form a consolidated pipe run, two purge dams are placed in the pipes, one in each pipe on either side of the weld zone. A purge gas can then be introduced into the area between the dams, usually by way of a needle injector inserted through a gap in the pipe end interface to be welded, to drive off the atmospheric contaminants.
Conventional purge dams for pipe welding applications must sometimes be placed relatively far from the weld zone so that they are accessible for removal following welding. In the example above where the ends of two pipe sections are welded together, the purge dams would need to be placed proximate to the ends of the pipes that remain exposed following welding. Such remote placement may require that large amounts of purge gas be introduced to fully decontaminate the weld zone.
To overcome the foregoing problem, it has been proposed that purge dams be formed using water soluble paper. The advantage of water soluble purge dams is that they can be placed in close proximity to a weld zone, and then removed following welding by dissolving them with water introduced through the pipe. Insofar as pipe systems typically undergo hydrostatic pressure testing or flushing with water prior to use, water soluble purge paper can be used in many cases without any additional processing steps following welding.
In some applications involving small diameter pipe, water soluble purge dams have been formed by simply crumpling a sheet of water soluble paper into a wad and inserting it into the end of a pipe to be welded. For larger diameter pipe, purge dams are conventionally constructed on site by forming a sheet of water soluble paper into a shaped dam structure. Following the construction of such a dam, it is inserted into a pipe end, and water soluble tape is used to affix the dam to the interior pipe wall. According to published descriptions of this technique, the formation of a water soluble sheet into the shaped dam structure entails cutting the sheet into a circle that is several inches larger than the pipe inner diameter, tracing or impressing the pipe inner diameter on the paper, and folding along this line to form a 90 degree lip that engages the pipe wall. The water soluble tape comes from a tape roll that is provided with the paper. After such a dam is placed in a pipe with the lip of the dam facing the open pipe end, several pieces of tape are torn from the tape roll and placed around the lip to secure it to the pipe wall.
A disadvantage of the foregoing do-it-yourself approach is that it requires field-formation of the purge dam to a desired shape, followed by labor-intensive taping of the purge dam to a pipe prior to welding. Paper, scissors and rolls of tape must all be carried to the welding location. In many cases, this could be remote from a supply shed or work shop where such materials are stored, requiring a truck roll to the location. Loss of valuable time will result if any of the required items are forgotten or used up while in the field. A further difficulty is that the structure to be welded may be relatively inaccessible. For example, a pipe structure at a refinery could be many feet above ground, such that it can only be reached by way of a ladder or scaffold. The difficulty and potential danger of carrying purge dam construction materials while climbing to such a location, and thereafter fabricating a purge dam while perched on the ladder or scaffold, are problems that welding personnel generally wish to avoid.
A prefabricated water soluble purge dam has been proposed as an alternative to the do-it-yourself approach described above. This publicized purge dam includes a sheet of water soluble paper that is preformed into a conical shape in which the open end of the cone conforms in diameter to the pipe to be welded. The purge dam is inserted into the pipe with the open end of the cone facing the open end of the pipe. Then, in accordance with the do-it-yourself technique described above, pieces of water soluble tape are torn from a tape roll and used to secure the open end of the cone to the pipe wall. Although this prefabricated water soluble purge dam concept is an improvement over the do-it-yourself technique, it still requires taping. Thus, a roll of tape must again be carried to the job site, perhaps carried up a ladder or scaffold, and pieces of tape must be torn from the roll and applied to the purge dam.
Accordingly, there is presently a need for improvement in the construction of water soluble purge dams. What is required in particular is a water soluble purge dam that is easy to install and which can be used with maximum convenience and safety by welding personnel who may be welding at a remote field location that is miles away from a supply shed or workshop, and potentially in a high risk environment.